In the medical field, doctors display medical images of subjects on a monitor and interpret the displayed medical images to observe the statuses or changes over time of lesions. Apparatuses (modalities) for capturing this type of medical image include an X-ray computed tomography (X-ray CT) apparatus, a magnetic resonance imaging (MRI) apparatus, a nuclear medicine diagnosis apparatus (single-photon-emission computed tomography (SPECT) apparatus and positron emission tomography (PET) apparatus), and an ultrasound imaging (US) apparatus. Each of the above-mentioned modalities is capable of providing an image of internal tissues of a subject by measuring a physical quantity specific to the modality. Images obtained by different modalities have different characteristics. Therefore, doctors generally make diagnoses using a plurality of modalities. For example, an image of a subject is captured by each of the MRI apparatus and the ultrasound imaging apparatus, and a doctor makes a diagnosis by comprehensively considering the obtained information.
However, in the case where images of the same subject are captured using a plurality of modalities, the obtained images are based on different coordinate systems. Therefore, a certain point within the subject may be displayed at different positions on the obtained images. According to the related art, the images captured by different modalities are not presented to the doctors such that the geometric relationship therebetween is visualized. The doctors estimate the relationship on the basis of the information of the images obtained individually by the respective modalities. Thus, there is a problem that the relationship between the images obtained by different modalities cannot be easily recognized.
Accordingly, attempts have been made to develop a technique for positioning the images obtained by different modalities with respect to each other and presenting the images to the doctors such that the doctors can easily recognize the relationship between the images. In particular, attempts have been made to develop a technique for presenting an ultrasonic tomographic image captured in an interactive manner and a three-dimensional medical image obtained by another modality in association with each other. In Patent Citation 1, an application is discussed which obtains an MRI image of a subject in advance, generates an MRI tomographic image which corresponds to an ultrasonic tomographic image being obtained, and displays the MRI tomographic image next to the ultrasonic tomographic image. According to this application, the relationship between the ultrasonic tomographic image and the MRI image can be easily recognized, and the efficiency and accuracy of diagnosis can be increased. In addition, there may be a case in which information, such as the position of a certain type of tumor, which cannot be easily visualized by the ultrasound imaging apparatus can be visualized in an MRI image. In such a case, the pieces of information of the images can compensate for each other. As a result, a puncture guide, for example, can be accurately operated.
To position the ultrasonic tomographic image and a three-dimensional medical image with respect to each other, the position of a cross section corresponding to the ultrasonic tomographic image is determined. The position of the cross section corresponding to the ultrasonic tomographic image can be determined by, for example, measuring the position and orientation of an ultrasound probe using an external sensor. According to Patent Citation 1, a sensor which measures the position and orientation using a magnetic field is attached to an ultrasound probe, and the position and orientation of the ultrasound probe are measured by the sensor.
A technique for positioning the ultrasonic tomographic image and a three-dimensional medical image, which are obtained by respective modalities, by using the information of the images has also been discussed. An example of such a technique is discussed in Non Patent Citation 1. According to this example, an ultrasonic simulation image is generated on the basis of a CT image captured in advance. Then, the relationship between the ultrasonic tomographic image which is actually captured by the ultrasound imaging apparatus and the above-mentioned ultrasonic simulation image is determined on the basis of the image information. Thus, the images obtained by the respective modalities can be positioned with respect to each other.
However, in the method using the external sensor, it is necessary to assume that the subject is a rigid body and does not change shape. However, it may be difficult to satisfy the assumption that the subject is a rigid body depending on the part of the subject to be examined. For example, in the case where a mammary area is observed for a breast cancer examination, it is difficult to assume that the part to be examined, i.e. a breast, is a rigid body. In particular, in an ordinary examination flow in which an MRI image is captured while the subject is in a prone (face down) position and an ultrasonic image is captured while the subject is in a supine (face up) position, the shape of the breast greatly changes due to the influence of gravity. In addition, since the ultrasonic image is captured while a probe is pressed against the part of the subject being examined, the shape of the part being examined also changes due to the pressure applied by the probe.
In contrast, in the technique according to the related art in which the images are positioned with respect to each other on the basis of the image information, the images can be positioned in consideration of the fact that the examined part of the subject is a non-rigid body by compensating for the change in shape between the images. However, since the change in shape has a very large number of degrees of freedom, only a local solution can be obtained. Therefore, it is difficult to obtain a correct positioning result.